As one of access schemes, a TDMA (Time Division Multiple Access) scheme is known. In the TDMA scheme, a beacon signal is periodically broadcasted and a period between two beacon signals (i.e., beacon interval) is divided into a plurality of slots. Each slot is allocated to each terminal. In the TDMA scheme, the terminal receives the beacon signal and transmits/receives only at the previously allocated slot. Accordingly, there is an advantage that the signal of each terminal does not collide with the signals of other terminals in the node-self allocated slot. On the other hand, each terminal has not necessarily data to transmits/receives at the allocated slot each time. In that case, the allocated slot is not employed, resulting in that the channel use efficiency lowers. In order to prevent the channel use efficiency's lowering, there is a method to devise a slot allocation scheduling of the TDMA. However, the method involves complexity of the scheduling processing.
In order to resolve the problem, the following related method is proposed: in a case that the terminal does not start the use of the allocated slot until certain timing from a head of the allocated slot of the TDMA, other terminal(s) can use the allocated slot of the terminal by a contention-based access using carrier sensing. The proposed method is adopted in the international standard IEEE 802.15.4e.
However, the related method does not assume a communication scheme in which two kinds of periods of a TDMA scheme period and a contention-based access scheme period are employed. The contention-based access scheme period includes a plurality of slots in each of which each terminal transmits/receives on the contention-basis at a slot unit.
Even if the related method is simply applied for the TDMA scheme period of the two kinds of periods in the communication scheme, it is difficult to increase the channel use efficiency because the contention-based access scheme period is not considered. In a case that emergency data, which is data required to urgently transmit, is generated, it is required to immediately transmit the emergency data. Therefore, sufficiently increasing the channel use efficiency is necessary.